In the testing of high current (above 10 Amperes) electronic circuit assemblies it is essential that a good electrical contact be established between the test apparatus test pins and the electronic circuit assembly test points. The electrical test pins must make good mechanical contact and be capable of withstanding high current to ensure low resistance to the test apparatus. While it is desirable to limit contact signal loss at any voltage, low resistance contacts are particularly advantageous at low voltages. At low voltages (below 5 volts) the loss of a substantial part of the available test signal voltage through test contacts can cause the test to fail. Additionally, for fast and flexible testing in various environments including, production, laboratory, burn-in, and repair, these connections need to be quickly established and quickly released.
Conventional circuit assembly test fixtures incorporate a test bed to make electrical contact at a plurality of points to the circuit assembly under test. The mechanical test probes in the test bed, called test pins, are used to make electrical contact with the circuit assembly at each circuit assembly test point, usually an exposed metal or solder tinned surface area, referred to as a pad.
When the circuit assembly is mounted in the test fixture, each test pin to contact pad connection can be characterized by electrical parameters. The resistance and inductance of the test connection are of particular interest for low voltage, high current signal test points where it is desirable to minimize both of these parameters.
As circuit assemblies to be tested are inserted and removed from the test bed, test pins must repeatedly make reliable mechanical and electrical connection between the test pins and the appropriate contact pads on the circuit assembly under test. These connections are made many times, particularly in the case of automatic test equipment.
Conventional test points incorporate a mechanical means to make and maintain contact with circuit assemblies of varying mechanical tolerances. Such test points, known as compliant test points, incorporate a sliding plunger supported by a spring in a hollow test point receptacle base. Each time a circuit assembly is placed in the test bed, the spring compresses as necessary to make mechanical contact between the compliant test pin and the test point contact pad. Referring to the drawings, FIG. 1 shows a test fixture 10 based on prior art compliant pins. Circuit assembly 12 is held in place by flange 11. The sliding plunger pins 14 extend to contact the circuit assembly pads 13.
Sliding plunger pin 14 is held against a test point pad 13 by the small force of spring 15, residing in the compliant pin hollow test point receptacle base 16. Since the spring 15 is located inside the hollow test pin base 16, it must be relatively small. Because of its small size, the internal spring 15 applies only a small force to the test pin 14. This small contact force results in a higher contact resistance than would be obtained with a stronger spring force.
In addition to the contact force, the resistance and inductance of the test connection are a function of the physical structure of the compliant pin. There are two electrical conduction paths within the structure of a compliant pin. The first is the direct path through any overlapping regions of the hollow cylindrical structure of the base receptacle and the plunger test pin. The second path is from the base receptacle through the spring to the test point. In parallel circuits, the combined resistance and inductance is lower than the either path alone, but the net resistance and inductance of this structure is still relatively high due to its small physical size.
An additional problem with the compliant test pin is that that the quality of the electrical connection deteriorates as spring and sliding plunger mechanically cycle with test circuit assembly insertion and removal. The increase in resistance is due to both a reduction in test pin spring force due to annealing and the deterioration of the surface contact between the test pin spring and the hollow base receptacle, and between the base receptacle and the test pin. Eventually, the resistance and inductance rises to a point where the voltage drop developed at the connection is so high that it precludes further testing without replacement of the failed compliant pin.
Accordingly, a need exists for a test instrument having low voltage, high current test pins that provide relatively constant low resistances and inductances that are stable over time and do not deteriorate with insertion cycles.